1. Field of the Invention
The present invention relates to an antiskid hydraulic pressure modulator for use in a hydraulic braking system of a vehicle for controlling hydraulic pressure applied to wheel cylinders.
2. Description of the Related Art
Known in the art are hydraulic braking systems with antiskid arrangements wherein the hydraulic pressure applied to wheel cylinders is controlled in accordance with varying ground surface conditions and vehicular conditions so as to stop a moving vehicle within a shortened braking distance while retaining adequate steerability of the vehicle.
An antilock control system disclosed in Japanese Examined Patent Publication No. 49-28307 includes a normally-open solenoid-operated input control valve disposed in a main hydraulic conduit connecting a master cylinder with wheel cylinders, and a normally-closed solenoid-operated output control valve disposed in a hydraulic pressure relief conduit connected between the wheel cylinders and a reservoir. By closing the input control valve and opening the output control valve, the brake fluid in the hydraulic braking circuit is released toward the reservoir thereby temporarily reducing the braking force to release any locking of the vehicle wheels. The brake fluid released to the reservoir is then re-charged by a hydraulic pump into the hydraulic circuit. The hydraulic pressure in the circuit is maintained at a constant pressure by closing both the input and output control valves, and the pressure is increased again by opening the input control valve with the output control valve closed. In this manner, antiskid control is achieved by controlling the hydraulic pressure in the braking circuit in three modes; the pressure relieving mode, the constant pressure mode, and pressure increasing mode.
It is also known to use a solenoid-operated three-position valve in place of the combination of an input valve and an output valve, as disclosed in FIG. 3 of Japanese Examined Patent Publication No. 51-6308. An example of such a solenoid-operated three-position valve is disclosed in Japanese Examined Utility Model Publication No. 58-17169.
In both cases, however, it has been recognized that, because the responsiveness of the solenoid operated control valves is inherently limited, the use of solenoid operated valves has been a bar to improve the overall responsiveness of the antiskid control system in such a manner that the vehicle can be stopped within a minimum braking distance. Another disadvantage of solenoid valves is that they are large in size and have a considerable weight. This has made it difficult to assemble all of the essential components of the antiskid system into a single unit.